Method for the calculation of fluid interface level

ABSTRACT

A method for calculating the boundary layer level between fluids, in particular the boundary layer level between oil and water in connection with the use of a segment-based boundary layer meter. 
     The calculations are based on the use of measurement signals from a number of segments, n, and the boundary layer level, L, of the water is calculated as follows: 
     
       
         
           
             L 
             = 
             
               
                 
                   
                     
                       ρ 
                       _ 
                     
                     - 
                     
                       ρ 
                       0 
                     
                   
                   
                     
                       ρ 
                       v 
                     
                     - 
                     
                       ρ 
                       0 
                     
                   
                 
                  
                 
                   ( 
                   
                     
                       L 
                       u 
                     
                     - 
                     
                       L 
                       l 
                     
                   
                   ) 
                 
               
               + 
               
                 L 
                 l 
               
             
           
         
       
     
     where:
     L u  Upper water range area (*)   L l  Lower water range area (*)   ρ n  Density of segment no. n from the profile meter   {circumflex over (ρ)} n  Corrected density for segment no. n     ρ  Calculated mean density   ρ v  Mean density of water from the profile meter   ρ o  Mean density of oil from the profile meter   N Total number of segments included

The present invention concerns a method for calculating the boundary layer level between fluids, in particular the boundary layer level between oil and water in connection with the use of a segment-based boundary layer meter.

In many different contexts, for example in a separator in a process system or production installation for oil and gas, in which water is an undesired component, it may be desirable to determine the level of the respective fluids (oil, water and gas) in the separator. The purpose of such a measurement may, for example, be to control the level in the separator. To achieve good level control, the level measurement must be “robust” and consistent. For level measurement of the water level in separators, however, this may be a challenge on account of emulsion.

In connection with the measurement of the level of fluids, it is possible to use a so-called profile meter that is designed to detect the boundary between the various phases (each fluid or the mixture of fluids). The profile meter consists of a large number of segments arranged in a vertical row above/below each other. Each segment is designed to detect a state or property in connection with the fluid to be detected and emits a signal to a registration/calculation unit. Such meters may, for example, be based on inductive measurement, capacitive measurement or radioactive sources.

For most prior art profile meters, it is difficult to detect changes in density for the respective fluid(s) or fluid mixture(s), which means that the measurements may contain a lot of “noise”. Furthermore, the measurements are not very continuous, as a standard algorithm used for the prior art solutions selects the “correct” segment for the phase change, based on the use of the registrations from 2 segments, for the level calculation by means of linear interpolation.

The present invention concerns a method for calculating the level or boundary layer between the different phases in a fluid based on the use of signals from several segments, thus producing much more reliable, more consistent calculations over time.

The present invention is characterised by the features specified in the attached independent claim 1.

The dependent claims 2-3 define advantageous features of the invention.

The present invention will be described in further detail in the following by means of examples and with reference to the figures, where:

FIG. 1 shows a simple drawing of a 150-segment profile meter used for the method in accordance with the present invention,

FIG. 2 shows a printout in the form of a bar chart of measurements from such a 150-segment profile meter.

As stated above, level control requires robust, continuous measurement with minimum noise. This can be achieved using measurement signals from several segments in a profile meter. FIG. 1 shows a profile meter 1 used for the method in accordance with the present invention. The profile meter 1 in the present example measures the density profile in a separator for gas, oil and water in a vertical direction with a “resolution” of 150 segments 2, i.e. the profile meter is based on the use of 150 segments arranged above/below each other.

A selection of these in the relevant area, for example for the water level, is selected. FIG. 2 shows a printout in the form of a bar chart of measurements from such a 150-segment profile meter arranged in a separator for oil, water and gas and shows the principle for selecting segments (shown as the shaded area) as the basis for the calculation. The selected area goes from the bottom over any sand layer to a little above the overflow to the oil outlet. Values for mean density are also shown in FIG. 2. The figure shows that the water has a mean density of approximately 1040 kg/m³ and the oil approximately 900 kg/m³. These values are used for calibration in the calculation of the boundary level for oil and water. The level is calculated by calculating the proportion of oil and water. The level lies right between these volumes, regardless of whether the oil and the water are separate or in the form of an emulsion. If there is an emulsion between the oil and water phases, the level will be calculated as the centre of the emulsion.

In order for the calculation to be correct, there may be no more than two components, oil and water (or a mixture of these components, an emulsion). This means that the correct measurement is not achieved if gas is registered in the highest segments and/or sand in the lowest segments. This problem can be avoided by defining “legal” values for the density of the segments. For instance, it is possible to set the following:

If density<oil (i.e. gas)->density=density of oil.

If density>water (i.e. sand)->density=density of water.

Density “corrected for gas and sand” is then:

{circumflex over (ρ)}_(n)=max(min(ρ_(n);ρ_(v));ρ_(o))  (1)

Mean density is calculated as follows:

$\begin{matrix} {\overset{\_}{\rho} = {\frac{1}{N}{\sum\limits_{n = 1}^{N}\; {\hat{\rho}}_{n}}}} & (2) \end{matrix}$

and the boundary layer level is calculated as follows:

$\begin{matrix} {L = {{\frac{\overset{\_}{\rho} - \rho_{0}}{\rho_{v} - \rho_{0}}\left( {L_{u} - L_{l}} \right)} + L_{l}}} & (3) \end{matrix}$

Where:

L Calculated water level

L_(u) Upper water range area (*)

L_(l) Lower water range area (*)

ρ_(n) Density of segment no. n from the profile meter

{circumflex over (ρ)}_(n) Corrected density for segment no. n

ρ Calculated mean density

ρ_(v) Mean density of water from the profile meter

ρ_(o) Mean density of oil from the profile meter

N Total number of segments included

The method in accordance with the present invention may easily be implemented in a suitable computer program and adapted to the registration equipment for the relevant segment profile meter in each individual measurement situation. The implementation may also easily take place directly in the automation system if the value of relevant segments is or can be made available.

*): (These ranges are defined within the segments on the profile meter.) 

1. A method for calculating the boundary layer level between fluids, in particular the boundary layer level between oil and water in connection with the use of a segment-based boundary layer meter, characterized in that the calculations are based on the use of measurement signals from a number of segments, n, and the boundary layer level, L, of the water is calculated as follows: $L = {{\frac{\overset{\_}{\rho} - \rho_{0}}{\rho_{v} - \rho_{0}}\left( {L_{u} - L_{l}} \right)} + L_{l}}$ where: L_(u) Upper water range area (*) L_(l) Lower water range area (*) ρ_(n) Density of segment no. n from the profile meter {circumflex over (ρ)}_(n) Corrected density for segment no. n ρ Calculated mean density ρ_(v) Mean density of water from the profile meter ρ_(o) Mean density of oil from the profile meter N Total number of segments included
 2. A method in accordance with claim 1, characterized in that density “corrected for gas and sand” is given by: {circumflex over (ρ)}_(n)=max(min(ρ_(n);ρ_(v));ρ_(o))
 3. A method in accordance with claim 1, characterized in that the mean density is given by: $\overset{\_}{\rho} = {\frac{1}{N}{\sum\limits_{n = 1}^{N}\; {\hat{\rho}}_{n}}}$
 4. A method in accordance with claim 2, characterized in that the mean density is given by: $\overset{\_}{\rho} = {\frac{1}{N}{\sum\limits_{n = 1}^{N}\; {\hat{\rho}}_{n}}}$ 